Among the processes for producing olefin polymers such as ethylene/α-olefin copolymers, a process using a Ziegler-type titanium-based catalyst composed of a titanium compound and an organo-aluminum compound is widely known. More recently, since the publication of a process of olefin polymerization in the presence of a metallocene-based catalyst which is composed of a transition metal compound such as zirconocene and an organo-aluminum oxy-compound (hereinafter which may also be referred to as “aluminoxane” described below) as a catalyst capable of producing olefin polymers with high polymerization activity [for example, Adv. Organomet. Chem. 1899 (1980), Angew. Chem. Int. Ed. Engl., 24, 507 (1985), etc.], improvements in the catalyst and the polymerization method have been carried out continuously by many researchers in the world. However, from industrial sectors, further improvements are desired from the standpoint of the polymerization activity, achieving of higher degree of polymerization, the amount of a comonomer introduced, or stereoregularity. Methods for solving such problems may be divided largely into two methods, that is, development of a new catalyst and improvement of polymerization conditions.
In the area of new catalyst development, olefin polymerization using a metallocene compound with a new, hithereto unknown structure that was bridged with a cyclopentadienyl ligand and a fluorenyl ligand was disclosed for the first time by J. A. Ewen [J. Am. Chem. Soc., 110, 6255 (1988)], and contributed significantly to the solution of the problem described above. On the other hand, as an example in the area of improvement of polymerization conditions, a process for producing an ethylene/1-hexene copolymer with a relatively high density by performing copolymerization using a metallocene compound containing a cyclopentadienyl group as a ligand under the conditions of high pressure and high temperature was disclosed [Japanese Patent Publication No. H01-503788]. Further, in Japanese Patent Publication No. H05-320246, a process for producing an ethylene/1-octene copolymer at high temperature using cyclopentadienylzirconium chloride and dimethylaniliniumtetra(pentafluorophenyl)borate was disclosed. However, in these disclosed processes, there was a problem that it was difficult to obtain a polymer with a high molecular weight in the low-density region (that is, in the region where the comonomer content is large). Also, further improvement in terms of polymerization activity is desired.